In a typical communications network, also referred to as a wireless communication network, communication system or wireless communication system, a User Equipment (UE), communicate via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device by which a subscriber may access services offered by an operator network and services outside operator's network to which the operators radio access network and core network provide access, e.g. access to the Internet. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket storable, hand held, computer comprised, or vehicle mounted user equipments, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
User equipments are enabled to communicate wirelessly with the network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core network nodes and possibly the Internet.
The radio access network covers a geographical area which is divided into cell areas, with each cell area, or group of cell areas, being served by a base station, e.g. a Radio Base Station (RBS), which in some radio access networks is also called evolved NodeB (eNB), NodeB, B node or base station. A cell is a geographical area where radio coverage is provided by the base station at a base station site. Thus, the communications network may also be referred to as a cellular network. The base stations communicate over the air interface with the user equipments within range of the base stations. The base station will be referred to as BS in some of the drawings.
A technology that is gaining ground in current communications networks is the use of Remote Radio Units (RRU), also known Remote Radio Heads (RRH). With this concept a base station is split into one part for the lower layer radio related mechanisms and another part for the higher layers, e.g. baseband processing. The former are called RRUs, RRHs or simply Remote Units (RUs), and are geographically distributed to the antenna sites, or close to the antenna sites, while the latter is deployed as a pooled resource cluster in a more central site, e.g. so-called baseband hotels. The centrally located part is called Main Unit (MU). Hence, this concept may also be referred to as RU-MU operation mode. RU-MU operation mode involves reduced hardware cost and reduced energy consumption, as a result of hardware pooling gains. A disadvantage of the concept is that RU-MU operation mode requires more transport network transmission capacity than regular operation mode and thus relies on the presence of (preferably cheap) high data rate transport network connections between the central site and the remote sites. Regular operation mode refers to a regular base station where the functionality of both the MU and the RU is integrated in the base station (in which case the terms “MU” and “RU” are not used).
A network deployment strategy which sometimes fits very well with RU-MU operation mode is the so-called heterogeneous networks deployment strategy, which is more or less generally assumed to be dominant in future communications networks. In a heterogeneous network large and small cells, high power and low power base stations/access points are mixed with each other in a largely overlapping fashion. Different cells may also employ different Radio Access Technologies (RATs), such as LTE FDD and LTE TDD, other RATs of the Third Generation Partnership Project (3GPP) family. Other RATs of the 3GPP family may be Universal Mobile Telecommunications System (UMTS)/High Speed Packet Access (HSPA) and/or Global System for Mobile communication (GSM)/General Packet Radio Service (GPRS)/Enhanced Data rates for GSM Evolution (EDGE), and non-3GPP RATs, e.g. the IEEE 802.11x family (where “x” may be e.g. “a”, “b”, “g” or “n”) RATs or 3GPP2 RATs, e.g. Code Division Multiple Access 2000 (CDMA2000). LTE is short for Long Term Evolution, FDD is short for Frequency Division Duplex and TDD is short for Time Division Duplex. IEEE is short for Institute of Electrical and Electronics Engineers.
In a heterogeneous network the same location is often covered (i.e. within the radio transmission and reception coverage area) by more than one cell and base station or access point. Pico cells, also referred to as the pico cell layer (or pico layer) are often deployed to provide greater capacity in hotspot areas with dense user population and intense wireless communication, while macro cells, also referred to as the macro cell layer (or macro layer) provide the overall wide area coverage. Pico and femto cells, as well as RRUs may also be deployed to improve coverage in locations which are poorly covered by the macro cell layer, e.g. indoors.
Another topic that currently receives a lot of attention in the communications network industry is the importance of energy efficiency, both for the purpose of reducing Operational EXpenses (OPEX) and in order to save the environment by reducing the CO2 footprint of communications networks. Another motive is to create goodwill for the operators and for the industry as a whole.
Combining heterogeneous networks and energy efficiency provides opportunities to leverage the greater number of base stations and the layered cell architecture of heterogeneous networks which often have overlapping coverage areas. To this end, schemes have been proposed where some cells of one layer, e.g. pico cells, are powered down (put in sleep mode) when the data traffic load and resource demands are low enough for the macro layer to handle. This principle may be used even in a non-overlapping scenario, where instead the coverage areas of neighboring cells are extended, either by tilting up the antennas or utilizing reconfigurable antenna system techniques or adaptive antenna mechanisms, so that they cover the area of a cell that is currently in sleep mode.
Sleep mode may be utilized on various time scales and various levels in terms of affected hardware. The sleep mode may affect the entire base station, the part of the base station responsible for a certain cell, the transmission equipment, including the Power Amplifier (PA), or single circuit boards or components being part of pooled equipment. The time scale may be hours, e.g. shutting down an entire base station during nighttime, all the way down to milliseconds, e.g. putting single components, or even the PA, into sleep mode on an LTE subframe basis. In order to carry information of different types between the base station and the user equipment, the LTE system has a defined LTE frame and subframe structure for the Evolved Universal Terrestrial Radio Access (E-UTRA), i.e. the air interface for LTE. A frame may have a certain length, e.g. 10 ms, and the frame is divided into a number of individual slots. A LTE subframe comprises two time slots.
Such energy saving sleep mode strategies may be utilized even in deployments where the coverage is not taken over by higher layer or neighboring cells. Potential scenarios may be low load periods (even empty cells which are not uncommon) or in combination with controlled Discontinuous Transmission (DTX) strategies where the user equipments in the cell know when to expect and when not to expect transmissions, such as reference signals and system information, from the base station. On-demand wakeup of sleeping cells to be ready to receive user equipments handed over from neighbor cells is another mechanism that may be used in conjunction with energy saving based on sleep mode.
A problem with the RU-MU operation mode concept is that the cheap high-rate transmission capacity in the transport network that the concept relies on is often not available. Instead, the transport network capacity is often more or less fully, or at least to a large degree, utilized during periods of high data traffic load. This limits the potential deployment scenarios for RU-MU operation mode and hence the full potential of hardware and energy saving cannot be leveraged.